1. Field of the Invention
The present invention generally relates to hearing aids. The invention, more specifically relates to hearing aids and to methods of acquiring and processing sound signals in hearing aids. The invention, still more specifically, relates to a hearing aid
comprising a housing, and at least a first and a second microphone for receiving ambient sound.
2. The Prior Art
It is a known problem that transducers, such as microphones, in electronic devices give rise to problems with wind noise.
In hearing aids there are two prime sources of this wind noise. One is the direct influence of the wind on the membrane of the transducer. Another is acoustic noise from turbulence around the head, the outer ear, and the housing of the hearing aid itself.
In an ear with normal hearing the first source is not very pronounced because the membrane, i.e. the eardrum, is buried deep in the inner ear at the end of the ear canal.
This is not the case in hearing aids. Conversely, in hearing aids the transducers are, for various reasons such as frequency characteristics, sound pressure sensitivity, directional phase characteristics etc., mounted as far in the open as possible in the attempt to achieve an optimum acoustic response of the transducer.
In many cases the wind influence directly on the membrane results in so large signals from the transducer to the signal processing circuitry in the hearing aid that saturation occurs. When parts of the processing circuitry such as analogue/digital converters (ADCs) in a digital hearing aid, amplifier stages in an analogue hearing aid or even the microphones themselves saturate, the output signal produced by the hearing aid will be impaired.
Attempts have been made to reduce the problems with wind noises in hearing aids.
US-A-2002/0037088 refers to several prior art attempts, and states inter alia that in order to reduce wind noise it is known to fit the microphone openings so as to protect them from the wind as much as possible. This is however generally incompatible with the desire of having the microphones mounted as far in the open as possible. In particular, the direction sensitivity, which is achieved by using two or more microphones at predetermined locations on a hearing aid, would suffer if the microphones were not in the open. Moreover, US-A-2002/0037088, does not elaborate on how to protect these openings, as it takes another approach to overcoming wind noises than external measures.
U.S. Pat. No. 3,875,349 provides a hearing aid possessing a first microphone having spherical sensitivity characteristics and a second microphone with directional sensitivity characteristics, and a switch for switching the amplifier selectively to either one of both microphones. The switch may be automatically controlled depending on the signal. This document seeks to resolve different listening situations and does not deal with wind influences.
U.S. Pat. No. 5,201,006 shows a hearing aid with a primary and a secondary microphone.
The primary microphone serves to provide the signal to be amplified. The secondary microphone is provided for feedback compensation purposes and is adapted to receive sound through a secondary duct, said sound mainly originating from parts of the shell between the shaped portion and the wall of the auditory meatus. As the purpose is feedback compensation, the secondary microphones are not intended to receive any ambient sounds.